1. Field of the Invention
The present invention relates to a method and apparatus for processing an audio signal and apparatuses using the same. More particularly, the present invention relates to a method an apparatus for eliminating shot noise, discontinuity, and a data loss and repairing an audio signal and apparatuses using the same.
The present invention is applied to compensation for loss of sound occurring when performing high speed reproduction (special reproduction) etc. in a receiver of a broadcasted audio signal, a reproduction system of an audio signal recorded on a magnetic tape, an optical disk, an opto-magnetic disk, or the like, and a decoding system of a digitally transmitted audio signal, for example, a Hi-Fi video apparatus, a digital video apparatus (digital video signal recording and/or reproducing apparatus), and an 8 mm video apparatus.
2. Description of the Related Art
In a receiver of a broadcasted audio signal, a reproduction system of an audio signal recorded on a magnetic tape, optical disk, opto-magnetic disk, or the like, a decoder of a digitally transmitted audio signal, etc., there is sometimes shot noise, discontinuity of data, and data loss—frequently occurrences in a communication path, a recording apparatus, a reproducing apparatus, packet communication, etc.
Such noise is generated everywhere due to, for example, noise generated in the air or in the apparatus, scratches or dust on the magnetic tape, scratches or dust on the optical disk, scratches or dust on an analog record disk, and reading error of the reproducing apparatus. Such noise causes a remarkably incongruity in sound.
In the past, when such noise and discontinuity occurred, the method of reducing the noise component by a low pass filter, a high pass filter, or the like and the method of replacing segments of data loss by the signal before and after them (Japanese Unexamined Patent Publication (Kokai) No. 9-274772) have been tried. Further, particularly, when digital data is used, the noise and the discontinuity have been reduced by means of prevalue hold, mean value interpolation, attenuation interpolation, muting, or the like.
However, a frequency filter distorts the normal signal portion as well and further is not that effective in elimination of short time and wide frequency band signals such as shot noise and discontinuity.
Further, prevalue hold and mean value interpolation are problematic in that a discontinuity newly occurs with the preceding and following data, so incongruity in sound is newly caused.
As concrete audio signal processing apparatuses, related art of Hi-Fi video apparatuses, digital video signal recording and/or reproducing apparatuses, 8 mm video apparatuses, etc. and problems thereof will be explained.
A Hi-Fi video apparatus using a magnetic tape has, as illustrated in FIG. 3, two tracks, that is, a fixed audio track and a Hi-Fi audio track, as tracks for recording an audio signal on the magnetic tape.
The fixed track is oriented parallel to the tape running direction and is provided at a position independent from the video signal track. It is scanned by a fixed head 17 illustrated in FIG. 2 and has a low dynamic range, a low frequency range, and a high noise level in structure. It is usually used for only monaural recording.
The Hi-Fi track is helically scanned by a rotary head drum 16 illustrated in FIG. 2 and records an audio signal by deep layer recording etc.* at the same position as the video track. It has a high dynamic range, a high frequency range, and a low noise level and is usually used for stereo recording.
In general, the Hi-Fi track is used for high quality sound reproduction at the time of normal reproduction, while the fixed track is used for low quality sound reproduction at the time of special (high speed) reproduction.
For example, Japanese Unexamined Patent Publication (Kokai) No. 5-292449 discloses a method of reproducing a video signal recorded on a video track at an (N+1)/N speed. However, the fixed track has to be used for reproduction of the audio signal. The reason for this is that, as shown in FIG. 4, at the time of special reproduction, the rotary head scans a plurality of Hi-Fi tracks by obliquely traversing them (helically scans them), so the video signal can be reproduced as one image by combining a plurality of fields, but, in contrast, noise due to discontinuity of the scanning of the rotary head frequently occurs in the audio signal to cause remarkable incongruity in sound, so the Hi-Fi track cannot be used. Therefore, the Hi-Fi track is not used. Instead, the fixed track is used using the fixed head 17—which is free from the problem of discontinuous scanning of the head.
In high speed reproduction for a head search or the like, in the case of high speed reproduction at 2× speed or more, a high quality of sound is not always required, so there is little problem.
However, when trying to save time while fully viewing and listening to the content by reproduction at a slightly high speed, for example, a 1.2× speed, the quality of sound when reproducing the audio signal recorded on the fixed audio track, which is fixed to such high speed reproduction, is insufficient. Therefore, utilization of the Hi-Fi audio track providing a high quality audio signal has been demanded.
In a digital video signal recording and/or reproducing apparatus using a magnetic tape, when giving as an example the format of for example a consumer digital video system, as shown in FIG. 24, a helical track inclined with respect to the tape running direction is divided into units of blocks. A compressed and encoded video signal is recorded at the center, while the audio signal and auxiliary signals are recorded on the two sides.
At the time of high speed reproduction in the digital video signal recording and/or reproducing apparatus, as shown in FIG. 25, a skip in the middle of a track is avoided by an auto-tracking mechanism. However, the discontinuity of the audio signal occurring due to the tracks not being read at the time of high speed reproduction causes a remarkable incongruity in sound at the time of reproduction in the same way as the case of a Hi-Fi video apparatus.
As the audio signal processing for reducing such incongruity in sound at the time of high speed reproduction in such a digital video signal recording and/or reproducing apparatus, various countermeasures have been considered heretofore. Examples thereof will be explained below.                (1) The method of permitting the discontinuity and connecting the signal as it is (Japanese Patent No. 2,687.706),        (2) The method of muting the discontinuous portion,        (3) The method of changing the speed of the rotary head to match the tape running speed and reading all data (Japanese Patent No. 2,766,065),        (4) The method of replacing the lost portions of the audio signal by the data before and after it (Japanese Unexamined Patent Publication (Kokai) No. 9-274772), and        (5) The method of connecting the audio signal before and after a lost portion by a cross fading (Japanese Patent No. 2,737,182).        
With the methods of (1) and (2), however, discontinuity or muting periodically occurs, so the incongruity cannot be solved.
With the method of (3), the apparatus becomes complex and further the data becomes too large, so time compression and sampling rate conversion are necessary. As a result, a new incongruity in sound such as a rise in sound pitch is caused.
With the method of (4), although there is the effect of reduction of the incongruity without muting the frames from which data has been lost, discontinuity still occurs before and after the replaced data. Accordingly, there is a problem of how to repair the audio signal without incongruity.
With the method of (5), although the discontinuity is solved by a cross fading of the data before and after the discontinuity, this is done without considering waveform periodicity etc., so there is a problem in that incongruity due to mismatch of phase is newly caused.
An 8 mm video apparatus recording and reproducing an audio signal by a rotary head suffers from problems similar to those described above.
A magnetic recording apparatus using not magnetic tape, but a fixed disk is excellent in random accessability, so the problem such as the skip of a track due to the physical structure described above does not occur. However, a certain time is required for reading data. Therefore, at the time of high speed reproduction, it is sometimes necessary to deliberately reduce the number of read fields. At this time, the problem of discontinuity of sound similar to the above arises.